UPSTREAM: drm/virtio: drop prime import/export callbacks

Also set prime_handle_to_fd and prime_fd_to_handle to NULL,
so drm will not advertive DRM_PRIME_CAP_{IMPORT,EXPORT} to
userspace.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190110111545.26768-1-kraxel@redhat.com


(cherry picked from commit b318e3ff7ca065d6b107e424c85a63d7a6798a69)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I604c266f985e52764924eb6deb5363bc7c01853e